It is conventional practice to drill an earthen borehole into the earth using a tubular string, typically called a drill string, extending from a rig at the earth's surface, and to cement a tubular string, typically called a casing string, in the borehole to prevent collapse and to stabilize the borehole. Some boreholes may be extended in a step-wise manner, e.g., with additional strings of casing cemented in the borehole as part of each step. Another tubular may be installed within the bore of the cemented casing string to facilitate, for example, the recovery of oil and/or gas from penetrated geologic formations.
Various actuatable devices may be coupled to a tubular and later actuated downhole to facilitate operations. For example, but not by way of limitation, bow spring centralizers may be used to position a casing string within a borehole, e.g., in a desired location therein, for the subsequent cementing step. Bow spring centralizers may be coupled to, e.g., disposed on, a casing at axially spaced intervals to provide an annulus between the casing and the borehole. Cement slurry may be displaced through the bore of the casing and into the annulus to form a protective liner. In boreholes having horizontal or highly deviated portions, more robust bow springs may be used to provide sufficient stand-off, but more robust bow springs may increase frictional resistance to movement of the casing through the borehole. It should be understood that more robust bow springs will more forcibly engage the wall of the bore in which the centralizer is disposed, and that the friction to movement of the tubular string is determined, at least in part, by the force of engagement of the bow springs with the wall of the bore.
One solution is to couple bow spring centralizers to the casing in a collapsed, e.g., retracted stand-off element(s), mode to reduce the frictional running resistance. The casing may be positioned in the borehole and the centralizers may then be deployed at the targeted interval to provide the desired stand-off. The centralizers are generally inaccessible because they are disposed within an annulus between the casing and the borehole. As a result, activating centralizers from a collapsed mode to the expanded mode, without compromising the integrity of the casing, presents a challenge.
One attempted solution provides a method of restraining a centralizer installed on a casing in a collapsed mode using one or more dissolvable restraining bands, and then dissolving the bands downhole using a strong acid, such as fluoric acid, circulated into the annulus. This solution is disfavored because the acid is dangerous to handle at the surface and can damage critical components in the borehole.
Another example of a device to be actuated after being positioned in a borehole is a packer. A packer may be used to seal an annulus between two tubulars such as, for example, an annulus between an installed casing and a production tubular disposed within the bore of the casing. The pressure in the annulus may be monitored so that a leak in the casing and/or production tubular can be readily detected, e.g., for diagnoses and/or repair. A packer may be coupled to a tubular string and run into a borehole in a retracted mode and then expanded to an isolating mode downhole. As above, a challenge is presented in actuating the packer from the retracted mode to the isolating mode without compromising the integrity of the tubular.
What is needed is an actuator that can be disposed on a tubular adjacent to an actuatable device, run into a borehole and reliably activated to actuate the device downhole without compromising the integrity of the tubular on which it is installed.